Pulse Width Modulation (PWM) is widely used in DC-DC power conversion and Digital-to-Analog (D-A) conversion. A pulse width modulated output signal from a PWM circuit, typically utilizing a central processor (CPU), comprises a train of pulses at a fixed frequency and having widths proportional to values of data samples. Thus, the PWM output signal may be considered as composed of a baseband signal and a carrier frequency. In order to filter out the carrier frequency from the baseband signal as in a D-A application, a low-pass filter is typically connected at the output of the PWM circuit (which may be viewed as integrating the PWM output signal into an analog signal). But, adding a low-pass filter will increase cost, which is not desirable in cost sensitive products such as consumer electronics.
An alternative, for example in an application for generating an audible sound from sample data values, to using a discrete low pass filter for separating the carrier frequency from the baseband signal is using a frequency shifting technique. In this alternative method, sample data values of the carrier frequency are generated at an increased frequency (also called reconstruction frequency) which is an integer multiple of the original frequency. For example, the reconstruction frequency may be four times the original frequency, so that for each sample value four pulses are generated with the same width proportional to the sample data value. The reconstruction frequency is chosen so as to be beyond the frequency response of an ordinary loudspeaker and the human audible frequency range, so that the audible sound heard from a loudspeaker to which the reconstructed PWM output signal is directly applied is the desired audio.
However, this frequency shifting technique increases the required processing tasks for generating the increased number of pulses and increases CPU update rate (thus shortening the Service Latency Time).
It is an object of this invention to provide a pulse width modulation circuit in which this disadvantage may be overcome or at least alleviated.